Current treatments for osteoporosis, such as the bisphosphonates, target osteoclast-mediated bone resorption. While these agents effectively reduce bone loss, this is complicated by an associated decrease in bone remodeling resulting in diminished bone formation and repair. Mounting evidence suggests that the presence of osteoclasts is required for bone formation by osteoblasts. Thus, the long term goal of this project is to identify potential therapeutic targets to inhibit osteoclast activity while preservin the anabolic effects of osteoclasts on osteoblasts and bone formation. One potential target to inhibit osteoclast activity without reducing the anabolic effects of osteoclasts is podocalyxin (PODXL). PODXL has previously been identified as a mediator of cell adhesion expressed by cells of the hematopoietic lineage. Preliminary data show that deletion of PODXL specifically in the hematopoietic lineage/osteoclast precursors (Vav/PODXLdel) leads to a high bone mass phenotype in female adult mice and increased osteoblast numbers. While these mice exhibit increased osteoclast numbers, resorption by these osteoclasts is impaired. Of particular interest, this high bone mass phenotype is evident only in female mice suggesting that the loss of PODXL is exacerbated by the presence or absence of specific sex steroids. Vav/PODXLdel bone marrow derived osteoclasts exhibit reduced activation of the Rac1 GTPase. Several groups have demonstrated that Rac1 activation is reduced by estrogen and this contributes to sex specific phenotypes in animal models. The central hypothesis of my project is that PODXL is necessary for optimal Rac1 activation; deletion of PODXL exacerbates the effect of estrogen to inhibit Rac1 activation, leading to impaired osteoclast resorption and a high bone mass in estrogen-sufficient female mice. My objectives are to characterize how the bone phenotype of Vav/PODXLdel mice is impacted by sex steroids in an in vivo mouse model and to define the mechanism by which PODXL modulates Rac1 activation and osteoclastic bone resorption. Understanding the mechanism by which PODXL facilitates Rac1 activation in pre-osteoclasts and how this is impacted by circulating sex steroids may lead to novel therapeutic targets to inhibit osteoclast activity without disturbing the anabolic effects of osteoclasts on the skeleton. Additionally, studying the role of sex steroids in the Vav/PODXLdel bone phenotype may elucidate mechanisms that contribute to postmenopausal and age-related bone loss.